Sound reproducing system



July 23, $935. J N JR 2,008,?7G4

SOUND REPRODUC ING SYSTEM Original Filed Jan. 22, 1931 3 Sheets-Sheet 1 lNVENTOR ATTORNEYS July 23, 1935. J HAMMOND, JR ZUSJQQ SOUND FEPRODUCING SYSTEM Original Filed Jan. 22, 1951 3 Sheets-Sheet 2 r l I Q g Q s aqmag umgvnuafly *qagvxag INVENTOR m M Wa;

ATTORNEY 5 Y 1935. J.'H. HAMMOND, JR 3 3 SOUND REPRODUCING SYSTEM Original Filed Jan. 22, 1931 3 Sheets-Sheet 3 11m! Valtage Z0 flmfl/Zer 94235 Jamil/WHO ATTORNEYS Patented July 23, 1935 SOUND REPRODUCING SYSTEM 7 7 John Hays Hammond, Jr., Gloucester, Mass.

Application January 22, 1931, Serial No. 510,382

Renewed September 22, 1933 13 Claims.

This invention relates to sound reproducing systems, and more particularly to an improved system for automatically expanding or multiplying the dynamic range of the sound energy.

" The invention relates particularly to a system of amplification in which a greater ratio of amplification or gain is produced for impressed signals of high intensity than for impressed signals of low intensity, whereby the volume range of the 16 signal is expanded.

One of the purposes of this invention is to provide a method by which the scratch and ground noises ordinarily produced in the recording and reproduction of music or speech may be greatly 'reduced or eliminated. Another purpose is to simplify the adjustment for a given expansion range and to simplify the construction of the jdevice. This is accomplished by the use of an amplifying system comprising a number of stages in 2b cascade, the gain ratio of a plurality of said stages being controlled by the output of a single control device in accordance with the input signal strength of a stage subsequent to the first controlled stage, with the control 'so exercised that the gain ratios of all controlled stages increase upon increase of signal input of the first controlled stage.

The invention also consists in certain new and. original features of construction and combinations 0' of parts hereinafter set forth. and claimed.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended here- -to, the invention itself, as to its objects and ad- 35" vantages, the mode of its operation and the manner of its organization may be better understood venience, but they are intended to be as generic.

in their application to similar parts as the art will permit. 7

Referring more particularly to Fig. l a standard phonograph mechanism 10 is provided with the usual turntable ll upon which rests a record l2. Engaging the sound groove of this record is the 55 needle of a phonograph pick up device it, the

electrical output terminals of which are connected to two terminals of a double pole double throw switch M. The other terminals of switch 14 are connected to the secondary of a transformer I5; the primary of which is connected through a batt/ery IE to a photoelectric cell I1. This cell is mounted behind a moving picturefilm l8, in front of which is an aperture plate I9 provided with an aperture 2 i' through which the light of a lamp Z2 is focused by me'ans'of a lens 23. The blades of' theswitch l4 are connected to a potentiometer 24, which is connected across the primary of a transformer the secondary of, whichiiscon nected in the input circuit of a space discharge amplifier 2E. The output circuit'of this amplifier is connected through a second transformer 2'l'to the input circuit of a second space discharge amplifier 28. A resistance 29 is connected in parallel with the primary of the transformer 21, in order to maintain the frequency characteristics independent of the amplitude characteristics. The secondary of transformer 21 is also connected through a stopping condenser 3| across a potentiometer 32, a variable section of which is connected througha leakage resistance to the input circuit'of a control device 34.

The filaments of the devices 26, 28 and Mare connected to one secondary Winding of a transformer 35, the primary'of which is connected to two conductors 36 and 31 which are connected through a switch 38 to a source of alternating current supply 39. The anode of the device 34 is connected through the primary of a transformer M to a point 42 on a voltage dividing resistor assembly 43. One end 44 of this assembly is connected through the primaries of transformers 27 and 62 to the anodes of devices; 26 and 28. The other end 511 of the resistor 43 is connected to one side ofa resistance 55, whichis connected in series with the secondary of transformers M and rectifier 59. Said resistance 55 is also connected in the control circuit of amplifiers 26 and 28 through the secondaries'of the two transformers 25 and 21. The resistance 55 is shunted by a condenser bank 55 which is made up of a plurality of condensers which, for example may. vary from .5 to 4 microfaradsf This condenser bank is controlled by means of a switch 51.. One contact 58 of this switch is connected to a point 49 of the resistor assembly 43. The filament of rectifier 59 isenergized by one of the" secondaries of the transformers 35, and the heated cathode is connected to the mid-pointof this secondary. The heated cathodes of the devices 26 and 28 are connected through an ammeter 6'!" to a point 46 of the resistance 43. The cathode of the device 34 is also connected directly to this point 46.

The output circuit of the amplifier 28 includes the primary of a transformer 62 which is shunted by a resistance 63, similar to resistance 29 above mentioned. The secondary of this transformer is connected to the input circuit of two space discharge amplifiers 64 and 65 which are connected in push-pull. Connected across the secondary of transformer 62 isa condenser bank 66- consisting of a plurality of condensers which, for example may vary from .00025 to .00125 microfarads. This condenser bank is controlled by means of a switch 67. The output circuits of the amplifiers 64 and 65 are connected to the primary of a transformer 68. The filaments of these devices are heated by a secondary winding. 69a of a power supply transformer 69, the primary of which is connected to the conductors Elli and 31. Across, the filament circuit ofthe devices 64. and. 65. is. a. resistance H... the midpoint of which. is connected. to. one end 44. of the resistor assembly 43. Rectifiers l2 and 13. have their cathodes. heated by secondary 55b and their space current supplied by secondary 69a of transformer 69. The positive-lead of the rectifiersis connected through two choke coils 7.4 and. 1.5 and. the primary of the transformer B& to the anodes of. amplifiers 64. and 65.. The negative lead: is connected to point 5.0. of resistor 43. Connected acrossthe output of the. rectifiers t2 and. 13 are three condensers l6, l1 and- 18- which, together with the two choke coils 1-4. and'l5 form a power filter of well known forna. Suitable by-pass condensers 45, 52 and 53.

. the field. of which is energized by a rectifier 82 which. is. supplied from. the secondary of a transformer 83, the primaryof which is connected: to the. conductors 36 and 31...

In the operation of the form of the invention. shown in. Fig; 1, when it is desired to play the phonograph the switch 14 isv thrown down, and when. it is desired to use the" moving picture apparatus the switch I4 is. thrown up. The switch- 3& is then. closed which starts the phonograph: mechanism ill, thus rotating the turntable H and record H in the usual manner. At the same time, the filaments of the devices 26, 2 8, 34, and 59 are heated by means of the energy passing thru the transformer 35'. The filaments of the devices [2. and 13 are heated by energy passing. through: part of. the transformer 59,. and the field winding. of the loud. speaker 81: is energized byenergy passing through the transformer 83 and rectifier 82-. which. rectifies the current, causing a direct current to. flow through the field winding.

rectifiers 1.2,. 13,. supply current to the anodes. of. amplifiers 64,. 65, and to the resistor assembly 43, which is connectedin series with thespace. current path of said amplifiers.

The resistor assembly supplies plate voltage for the devices 25 and. 28. from point 4.4, and plate voltage for the. device 34 from point 42.. Biasing voltage for. amplifiers 2.6 and 28 is supplied from point 5.0. through resistance 55. The amount of bias is addustable by resistance contact 41,

which maybe connected to point 50., to prevent loss. of energy inthat part of the resistance between. 41 and 5D. This is possible by the substantially constant current which fiows through the bias resistor assembly.

With no applied signal, the voltage on the grids of the devices 26 and 28 is determined by the setting of the movable contact 41 on resistor 43, which is so positioned that a small plate current will flow through these devices as indicated by the reading of the meter 6|. When a signal is received from either the phonograph pick up l3 or the photoelectric cell I1, energy will pass through the transformer 25 to the input circuit of the device 26 where it will be amplified and passed through the transformer 21 to the input circuit of the device 28. Part of this amplified energy will pass through the potentiometer 32 and leakage resistance 33 to the input circuit of the device 34. The output of this device passes through the transformer 4| and is rectified by the device 59. which is preferably a substantially linear rectifier. This rectified current passes through the resistance 55 in the direction of the arrow, thus causing a potential difference to be built up. across this resistance which is proportional to the current flowing through it and, because of the linear nature of rectifier 59,. substantially proportional to the input signal strength. to control. device 34. This potential difference will oppose the biasingv voltage of the devices 26 and. 28 as determined by the setting of the contact 41,. thus decreasing the bias on these devices, which. in turn decreases their impedance and, therefore, increases the amplification. of the stage in which they are connected.

It is thus seen that as the input signal strength increases, the amplification factor of the stages including devices 26 and 28 will. be increased. Inthis way, the amplification: factor is made small for weak signals and large for strong signals. The output of the device 2& passes through the transformer, 62. to the input circuit of the pushpull amplifier, comprising the devices 64 and '65- where it is amplified and fedthrough the transformer 68' to the moving. coil of the loud. speaker- Bil where it. is reproduced as speech or music in the usual way.

The potentiometer 24. controls the input signal strength. and; therefore controls the total volume of the system. The potentiometer 32 controls the signal strength applied to the grid. of the device 34 with respect to the signal strength. applied to the grid of the device 28,,and in this. way

controls the dynamic range of the system, so that when. the potentiometer 32 is set to pick up a large amount of energy the dynamic range of the system will be large, and when the potentiometer is set to pick up a small. amount of energy, the dynamic range will be small. It is to.

be understood that potentiometers 24 and 32 may be operated simultaneously or independently as may be desired.

The leakage resistance 33 is placed in. the input circuit of the device 34 so as to limit the output of tube 34 and consequently of rectifier 59 and thus to prevent the bias on tubes 26 and 28 becoming so small as to cause tone distortion of the output of the system.

The condenser bank 56 is used to control the time rate of decay or decrescendo. If the music is rapid and a quick response at the termination of' the signal is desired, a small condenser is connected across. the resistance 55 thus causing a rapid recovery; while if the music is slow,, as in organ recitals, av large condenser is used which causes a long d'ecrescendo.

If it is desired to eliminate the dynamic multiplicatijon entirely, the switch 5.! is thrown on the contact. 58 thus applying a. suitable negative voltage from point 49 directly to the grid return connection of the devices 25 and 28 which then operate as normal amplifiers.

The condenser bank 85 is used as a tone filter and provides a further control for the scratch attenuation. A

In thisinvention it is necessaryto-shunt the primaries of boththe transformers" 2i and 62 by resistances 23 and 63 so as to maintain the frequency characteristics of the amplifier stages independent of their gainratio characteristics.

Fig. 2 is a curve depicting the decibels attenuation with different output volumes as measured by the plate current in meter 6!. For example, an output volume from loudspeaker 8| corresponding to a plate'current of five milliamperes in meter. 6i may represent a three decibel loss from the normal or optimum output due, for example, tofilter 6?.) ."Minimum output volume corresponding to a plate current of one milliampere may represent a loss of thirteen decibels from normal or optimum output. To obtain this relation, contact 41 may be adjusted to give a plate current of one milliampere with no signal. As the grids of the amplifier tubes 26 and 28 swing more positive due to the signal causing a positive potential across biasing resistance 55, when approximately maximum or normal output is reached, the attenuation will reach about three decibels loss. Thus from approximately normal or maximum output to approximately minimum or zero output, the scratch attenuation increases ten decibels.

Fig. 3 shows the relationship between the input voltage to the amplifier and the bias on the grid of the amplifier. It is seen that this curve follows approximately a square law, so that when the input voltage is low a considerable increase therein will only produce a small decrease in bias, so that for weak signals the amplification factor is very low. With hi h input voltage a small increase therein produces a considerable decrease in bias, so that for strong signals, the amplification factor is very high.

The shape or" this curve is due to the fact that the input voltage for the control device is taken oii subsequent to one of the controlled amplifiers (after amplifier tube 25), which causes the curve to follow approximately a square law.

Although only a few of the various forms in which this invention may be embodied have been shown herein, it is to be understood that the invention is not limited to any specific construction, but might be embodied in various forms without departing from the spirit of the inven tion or the scope of the appended claims.

What is claimed is:

1. A system for reproducing sound from a sound record comprising a plurality of stages of amplification, each stage including a space discharge amplifier having a control element, an impedance in series with said control elements, said impedance being common to all of said stages, and means for passing a current through said impedance, said current increasing with increase in strength of the input signal, said current being in a direction such that the potential drop in said impedance opposes the normal bias of said control elements, the relationship being such that an increased signal strength decreases the normal negative bias of said control elements and causes an increase in the amplification ratio of the space discharge device.

2. A system for reproducing sound from a sound record comprising a plurality of stages of charge amplifier having a-control elementg an impedance-in series with said control elements, said impedance being common to" all of said stages, means for passing a current through said impedance proportional to the strength of the input signal and in a direction such that thepo-' tential drop in said impedance opposes the normal negative bias of said controlelement's, the relationship being such that an increased'signal strength'decreases the normal negativebias of said control elements, and causes an increase in the amplification ratio of the space discharge devices, and a variable capacitance connectedin parallel with said impedance and bearing such relation thereto that'the current passing through said impedance is proportional to the averagevalue of the inputsignal over a predetermined period of time. r 7

3. A method of operating a space discharge amplifier for reproducing sound from a sound record comprising a plurality of stages of amplification which comprises causing the intensity of the input signal to vary the amplification factors of all of said stages in such manner that an increased signal increases said amplification factors whereby the volume range of said amplifier is expanded.

4. A system for reproducing sound from a sound record comprising a plurality of amplifiers arranged in cascade, a resistance common to all of said amplifiers and in series with their control elements, means for passing a current through said resistance which is dependent upon the input signal strength of one of said amplifiers whereby the volume range of each stage of said amplifier is expanded.

5. A system for reproducing sound from a sound record comprising a plurality of stages of amplification each of said stages having a variable amplification factor and a single means for controlling said ampliflcation factor, said means being automatically operable in response to variations in signal strength, said control being adapted to cause the amplification factor to vary as an increasing function of said signal strength whereby the volume range of the signal is expanded.

6. A system for reproducing sound from a sound record comprising a plurality of stages of amplification, a control means for controlling the amplification factor of all of said stages, said means comprising an impedance carrying a current changing with the average value of the signal strength and a signal rectifier operated in accordance with the signal strength for controlling the current which is passed through said impedance.

'7. In a system for reproducing sound from a sound record including a plurality of space discharge amplifiers, a control circuit for said amplifiers operable in accordance with the average signal strength and adapted to control the gain ratio of said amplifiers, means for varying the timing of said control circuit, means for rendering said control circuit inoperative ,and a single rotary switch for controlling both of said last mentioned means.

8. In a system for reproducing sound from a sound record including a plurality of space' discharge amplifiers in cascade relationship, a control circuit for varying the amplification factor of. said amplifiers and means for operating said control circuit in accordance with the signal strength at an intermediate point between the controlled stages.

, 9 In a; system or reproducing sound, a sound record, atpick-up device therefor, a. multi-stage amplifier fed by saidpick-up device, a translating device, fed: by said amplifier, a control circuit fed by theoutput. of the first stage of said amplifier for controlling, the gain ratios of both of said stages.

10-. In a system for-reproducing sound, a sound. record, a pick-up device therefor, a multi-stage amplifier fed by said pick-up device, a translating device fed by said amplifier, and a control circuit fed by said pick-up device for controlling, the gain, ratios of both of said stages.

1L. In a system for reproducing sound, a sound record, a pick-up device therefor, an amplifier fed: by said pick-up device, a sound propagating receiver fed by said amplifier, a control circuitfor controlling the gainv ratio of said amplifier, means in said control circuit for rectifying, the signal from said? Dickup, device andimpressing the same across a biasing resistance anda variable capacitance across said biasing resistance, to vary the timing of the dynamic action.

12. In a. system for reproducing, sound, a sound record, a pick-up device therefor, an amplifier fed by said pick-up, device, a. translating device fed by said amplifier, meansfor controlling the amplification ratio of said amplifier responsive to the volume output thereof, and means for adjusting the range of said controlling means.

13. In a system for reproducing sound from a sound record, apluralityof amplifying means having variable amplification ratios, and a single means to control the amplification ratios of all of said amplifying means, said control means being responsive to the average energy received from said sound record.

JOHN HAYS HAMMOND, JR. 

